The present invention relates to an image forming method for developing an electrostatic latent images formed on the surface of an image-bearing member by using a magnetic developer held on the surface of a cylindrical developer conveying member made of a permanent magnet.
Conventionally, an image forming method in which an electrophotography or electrostatic recording is applied to a printer, facsimile machine, and other printing devices, is comprises steps as follows: forming an electrostatic latent images on the surface of an image-bearing member drum shaped, for example, attract magnetically and convey the magnetic developer on the surface of a developing roller provided opposite to this image-bearing member, and comprising a non-magnetic sleeve and a permanent magnet inside the sleeve and relatively rotatable. Thereafter, with the formation of a magnetic brush in the developing region, rub the electrostatic latent image formed surface on said image-bearing member with this bush, thereby the elctrostatic latent image is visualized as a toner image. It is the most common step to heat-fix the developed toner image after transferring it onto transfer sheet such as plain paper.
In a development and fixation means as above, since not a small amount of toner remains on the image-bearing member after transferring the toner image onto the transfer sheet, a cleaning device is ordinarily provided to remove the residual toner. From this arises a problem in that a space for said cleaning device must be secured around the image-bearing drum, thereby hindering the entire structure inclusive of a printer from being made compact.
Known as one example of means for solving the problem and aiming at the miniaturization of the whole device are the omission of said cleaning device and the provision of a so-called developing and cleaning unit for removing residual toner remaining on the image-bearing member after transferring as well as for developing an electrostatic latent image in the developing region where the image-bearing member and developing roller are opposed to each other.
In the conventional developing method mentioned above, since a magnetic developer is attracted magnetically and conveyed on the surface of a sleeve positioned outside the permanent magnet and a magnetic brush is formed on the surface of the sleeve in the developing region, the magnetic developer is conveyed through a frictional force with the surface of a sleeve, and its conveyability and the shape stability of a magnetic brush are mainly depended on a frictional coefficient of the surface of the sleeve.
Consequently, the surface of the sleeve is ordinarily formed into a rough surface, but is worn out and becomes smooth during use of long period and therefore the frictional coefficient changes with the lapse of time or sometimes changes locally. Not only the conveyability but also the shape and/or stability of a magnetic brush thus changes. Thus, when performing one development per turn of the image-bearing member by using a magnetic brush in said developing and cleaning unit, no residual toner is completely removed if there is a residual toner after transferring of a toner image onto tarnsfer sheet on the image-bearing member and residual toner may deposite to and remain on the previous electrostatic latent image formed area even after developing.
There is a problem in that such poor removing of a residual toner as described above considerably deteriorates the quality of an obtained image. Therefore, to solve such a problem, there is also a scheme for the complete removing of said residual toner by using a step of performing one development for two turns of an image-bearing member. However, this method has a problem in that the formation speed of images inevitably decreases and the requirement for speedier image bearing cannot sometimes be met.
Meanwhile, to meet a recent increase in the need for a higher quality image, there is a tendency for toner to become smaller in size and, consequently, a mean particle size must be formed of 4 to 9.mu.m. Known as an ordinary method for the production of toner is the method by grinding and classifying a raw material after heating, kneading, and cooling. However, a small-particle size toner produced by the aforesaid method is disadvantageous in that it has low fluidity due to the non-sphere shape of particles. Thus, such a developing and cleaning station as described above has a drawback in that it is difficult to completely remove residual toner after transferring.
Incidentally, when a large amount of fluidity-improver such as fine particles of silica is added to improve the fluidity of the aforesaid small-particles size toner, in spite of improved fluidity, disadvantages occur in that the surface of an image-bearing member is damaged and a large change in charge quantity of toner due to a change in humidity becomes larger.
A conventional method for forming a color image has been carried out as follows:
FIG. 6 is an explanatory drawing of the main constitution showing one example of a color image forming device without the use of a conventional intermediate transfer medium. In FIG. 6, the image-bearing drum 101, including an image-bearing surface (not shown) shaped like a cylinder and comprising photosensitive layer such as zinc oxide or organic semiconductor on the peripheral surface, is provided so as to be rotatable in the direction of the arrow. Developing unit 102Y, 102M, 102C, 102BK contain magnetic developer comprising yellow, magenta, cyan, and black toners, respectively, at the opening of which a rotatably installed developing roller 103 is disposed near said image-bearing drum 101.
Most generally, the developing roller 103 comprising a coaxially inserted and relatively rotatable composite of a permanent magnet with a plurality of axially extending magnetic poles provided on the peripheral surface and a sleeve shaped like a hollow cylinder made of non-magnetic material, for example, is for developing an electrostatic latent image formed on the surface of said image-bearing drum 101 attracting magnetically the magnet developer on the surface of the sleeve and forming the magnetic brush.
Near the peripheral surface of the image-bearing drum 101 are provided a charger 104, a transfer means 105, and a cleaner 106. Downstream of the transfer means 105 in the paper feed path 108 is provided a fixer 107, comprising a pressure contact and rotatable composite of a heat roller 107a and a pressure roller 107b.
According to the constitution described above, the image-bearing drum 101 rotates in the direction of the arrow, the surface of which is uniformly charged by means of the charger 104, for example illuminating an exposure L corresponding to image information for yellow, the first color, leads to formation of electrostatic latent image. When this electrostatic latent image reaches the developing unit 102Y, the developing unit 102Y begins to function and yellow toner is supplied to said electrostatic latent images by the developing roller 103 so that a yellow image is formed.
The image-bearing drum 101 carrying said yellow image successively passes the inactive developing unit 102M, 102C, and 102BK, the transfer unit 105, and the cleaner 106, then charging with the charger 104 and illuminating an exposure L corresponding to an image information of magenta, the second color, leads to formation of an electrostatic latent image in magenta. From this electrostatic latent image, a magenta image is formed by using the developing unit 102M for holding a magenta color. Further, after a similar process, each individual colored image is superimposed to form a full color image on the image-bearing drum 101.
When said color image reaches the transfer unit 105, paper P is supplied simultaneously with this and the color image is transferred to the paper P, then is fixed by means of the fixer 107. The residual toner remaining not yet transferred to the paper P at the transferring, is removed from the surface of the image-bearing drum 101 by using the cleaner 106, followed by the next image formation process.
As said conventional magnetic developer to be used for a color image formation is used a two-component magnetic developer, mainly comprising a non-magnetic color toner and magnetic carrier. However, the two-component magnetic developer to be used in this case must: be controlled to maintain the toner concentration within definite limits and, accordingly, a toner concentration control device must be provided in each one of the developing unit 102Y, 102M, 102C, and 102BK. Although an ordinary image forming device has a single developing unit, a color image forming device needs four developing unit. Thus, there are problems in that the provision of a toner concentration control device in each developing unit complicates not only the constitution of the whole apparatus but also maintenance and manipulation and further hinders miniaturization and cost saving.
Formation of a color image without use of an intermediate transfer medium must proceed from one process to another for the superimposition of individual colored images and consequently, undesired toner may attach to the nor-image area, thereby lowering the image quality. To solve this drawback, a so-called non-contact developing process is proposed wherein a magnetic brush kept away from contact with the image-bearing drum 101 as an image-bearing means have been proposed, e.g., Japanese Patent Laid-Open Publication No. 216324/1993.
However, conventional devices including that proposed above have drawbacks in that the constitution of the developing roller 103 to insert a hollow cylinder-shaped sleeve outside the permanent magnet as a magnetic field generation means lowers the magnetic flux density on the surface of the sleeve, thereby reducing the magnetic attraction force of the magnetic developer.
Also, an arrangement in which the permanent magnet is fixed and magnetic developer is magnetically attracted and conveyed by a rotation of the sleeve has problems in that the wearing of the surface of the sleeve causes the conveyability to vary in time series or locally, thereby bringing about a shape instability of the magnetic brush and also an unstable developing ability.
Further, since the thickness of the developer layer increases and a magnetic brush happens to stand up near the magnetic poles of permanent magnet, magnetic developer to be attracted and conveyed on the surface of the sleeve has drawbacks in that toner in magnetic developer scatters and mixes with other-colored toner, thereby lowering the image quality. A further problem lies in that, to prevent the scattering of toner and contamination of colors, the concentration of toner must be kept lower than, say, 7 weight %, thereby bringing about lowering of the image density.
Recently, requirements for the miniaturization of devices used for image formating process have been intensified and it has become important to miniaturize the developing unit. As a means for satisfying these requirements, it has been proposed to attract magnetic developer directly on the surface of permanent magnet member without using a sleeve and to convey magnetic developer by a rotation of a permanent magnet member (e.g. Japanese Patent Laid-Out Publication No. 201463/1987).
FIG. 5 is an explanatory drawing of the main part showing one example of sleeveless type developing unit as described. In FIG. 5, a developing container 201 contains a magnetic developer 202 mainly comprising, say, toner and a magnetic carrier, under which a permanent magnet 204 is rotatably provided. The permanent magnet 204 is formed in such manner as to be conductive at least on the surface, on the peripheral surface of which a plurality of axially extending magnetic poles is provided in a cylindrical form.
Said permanent magnet 204 can be formed of a resin bonded magnet comprising a mixture of ferromagnetic powder and resin (cf. Japanese Patent Laid-Open Publication No. 130407/1982, No. 905/1984, and No. 226367/1984). As a means for making the surface conductive, it is allowable to form a conductive layer on the surface by plating for example, or to add an electro-conductive powder during kneading of raw materials. It is also possible to make the permanent magnet 204 semiconductive by making it of a hard-ferrite magnet.
An image-bearing drum 203, formed in such a manner as to be rotatable in the direction of the arrow, is opposed to the permanent magnet 204 via a gap g. A doctor blade 205, provided in the developer vessel 201, is opposed to the permanent magnet 204 via a gap t and serves to regulate the layer thickness of magnetic developer 202 to be attracted on the surface of the permanent magnet 204. A charging roller 206, a transfer roller 207, and a cleaning device 208 are disposed around the image-bearing drum 203, with the surface of which a doctor blade 209 is in contact. Incidentally, to magnetic developer 202 to be attracted on the permanent magnet 204 is applied via the permanent magnet 204 or the doctor blade 205 a bias voltage supplied from the DC power supply (not shown).
According to the constitution described above, on rotating in the direction of the arrow the image-bearing drum 203, charging roller 206, permanent magnet 204, and transfer roller 207 individually, the surface of the image-bearing drum 203 is uniformly charged by means of the charging roller 206. On illuminating an optical signal (not shown) onto the image-bearing drum 203, an electrostatic latent image is formed. When magnetic developer 202 is attracted onto and conveyed by the permanent magnet 204 and gets to the developing region opposite the image-bearing drum 203, toner in the magnetic developer 202 is put in the electric field of an electrostatic latent image formed on the image-bearing drum 203 and the electrostatic latent image can be developed.
The developed toner image is transferred to paper P by means of the transfer roller 207, moves in the direction of the arrow, and is fixed. Residual toner that remains on the image-bearing drum 203 after image transfer, is scraped off with a cleaning blade 209 in rubbing contact with the surface of the image-bearing drum 203 and is collected in the cleaning device 208.
Used as a two-component magnetic developer in the image formation device described above is magnetic developer 202 comprising a mixture material of, say, magnetic toner with a particle size distribution between 5 and 20 .mu.m and a ferrite carrier with a particle size distribution between 70 and 140.mu.m. Incidentally, there are also cases where non-magnetic toner is used in place of magnetic toner.
In these cases, the use of small particle size toner is needed for formation of a fine image. There is a problem in that, in using large-particle size magnetic carrier as described, the poor capability of giving electric charge to toner is apt to induce the generation of fog, thereby lowering the quality. Accordingly, not only to obtain a fine image, but also as magnetic carriers, small-particle size toner is required.
However, for the conventional development roller with a sleeve, by use of said small-particle size magnetic carriers, the carrier is likely attach to the image bearing member, leading to a decrease in quality. For an image formation means of such a type as directly attracts and conveys magnetic developer 202 by using the permanent magnet 204 with omission of such a sleeve as shown in FIG. 5, although no carrier attaching occurs, unlike said developing roller with the sleeve, there are other problems in that the use of magnetic carriers having a high magnetic force increases a rotation torque for driving the permanent magnet 204, which is apt to induce spent phenomena that the resin component constituting toner adhere to the surface of magnetic carriers, thus leading to a reduction in the life of magnetic carrier and the generation of fog.
Furthermore, a conventional image formation means has combined problems in that, since toner concentrations in a two-component magnetic developer is 3 to 5 weight % for non-magnetic toner and lies in a relatively narrow range of 20.+-.5 weight % for magnetic toner, a toner concentration control means such as a toner concentration sensor is needed, resulting in low operability and a complicated device.
In addition, when developing an electrostatic latent image by means of said developing roll of a sleeveless type, the use of small-particle size toner, such as 4 to 9.mu.m in average particle size, is preferred for toner containing in the magnetic developer 202. In recent years, a fine image is required, so that the particle size of toner must be still smaller.
It is known as a general method for the production of toner, to grind and classify blended raw materials after heating, kneading, and cooling. However, the production of small-particle size toner by such a method has problems in that, since a long time is needed for the grinding process, a possible mean particle size is limited to around 7 mm and in that the production work is complicated and the production cost rises.
For these reasons, methods by polymerization have been proposed for the production of small-particle size toner, which polymerization process includes suspension polymerization and disperse polymerization process. These methods permit small-particle size toner to be produced with comparative ease, magnetic developer suitable for said developer of the sleeveless type to be obtained, and a fine, and high-resolution image to be formed.
However, toner produced by an ordinary polymerization process is restricted to that of spherical particles, has difficulty in the uniform dispersion of coloring agent and other blend components into toner particles, and cannot satisfactorily meet the required properties for toner. In particular, toner particles formed in a spherical shape are so large in fluidity that they often pass between the image bearing member and blade in the blade cleaning means representative of an ordinary cleaning scheme, thus entailing a problem in incomplete removal and collection of the residual toner on the surface of an image-bearing drum 203.